Using GHC API.

Lang.hs

@@ -1,14 +1,7 @@
 module Lang where
 
-import Control.Applicative
 import Param
-import System.IO
-import System.Process
+import DynFlags
 
 listLanguages :: Options -> IO String
-listLanguages opt = convert opt . lines <$> getLangs opt
-
-getLangs :: Options -> IO String
-getLangs opt = do
-  (_,hout,_,_) <- runInteractiveProcess (ghc opt) ["--supported-languages"] Nothing Nothing
-  hGetContents hout
+listLanguages opt = return $ convert opt supportedLanguages

ghc-mod.cabal

@@ -28,7 +28,7 @@ Executable ghc-mod
     GHC-Options:        -Wall -fno-warn-unused-do-bind
   else
     GHC-Options:        -Wall
-  Build-Depends:        base >= 4.0 && < 5,
+  Build-Depends:        base >= 4.0 && < 5, ghc,
                         parsec >= 3, process, haskell-src-exts,
                         directory, filepath
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